Distraught: Isabel's parents, Sergio and Becky Celis, have made several emotional pleas for their daughter's safe return

Hunt: The investigation into Isabel's disappearance so far has cost the Tucson Police Department more than $1million

Acting on some of the 1,000 tips they have received so far, police have also searched homes in the family's neighbourhood, dug through a nearby landfill and scoured waterways and drainage systems in Tucson.

So far, the investigation that has involved 20 detectives and 30 staffers has cost the Tucson Police Department more than $1million, Gatherhas reported.

In the days after the disappearance, Isabel's parents have publicly spoken out several times, making emotional pleas for the safe return of the little girl.

‘Just please, please, to the person or persons who have Isabel, tell us what you want. We will do anything for her. We're looking for you, Isa,’ Sergio Celis said tearfully on one occasion.

Friends and co-workers of Becky Celis, who is a nurse, have distributed more than 250,000 fliers in the Tucson area and in Nogales, Arizona, in an effort to find the missing girl.

The search for Isabel has extended into the Mexican state of Sonora, where authorities have been circulating her photos

The
search for Isabel has also extended into Mexico, where authorities in
the state of Sonora have been circulating photos of the first-grader.

Police
began looking at the possibility that Isabel could have been taken
there because of Tucson's close proximity to the border, Lieutenant Fabian
Pacheco told ABC News last month.

Isabel is 4ft tall, with brown hair and hazel eyes. She weighs approximately 44lbs. She was last seen with her hair in two long braids. She is missing one front tooth and one bottom tooth.

In late April, police released surveillance video showing five men leaving a Tucson nightclub and walking past the Celis home on the
night when Isabel vanished. Later, however, police determined that the footage was not connected to the case.

Now, authorities say they are reviewing hours of new footage that they have obtained in hopes that it will lead them to new clues.

Meanwhile, the 911 call that was made the day Isabel disappeared
continues to be withheld from the public. It will be released after
police remove some of the information in the call, according to the Arizona Daily Star.